Subtopics - Magnetic Effect of Current (NEET)
Magnetic fields from currents, forces on charges and conductors, and electromagnetic devices
1) Biot-Savart Law and Magnetic Field Configurations
Biot-Savart law for the magnetic field due to a current element, direction rules (Maxwell's cork screw, right-hand thumb rule), magnetic field due to a straight conductor (finite, infinite, semi-infinite), circular current loop (centre, axial point, arcs), Helmholtz coils, and concentric coplanar loops.
2) Ampere's Circuital Law and Its Applications
Ampere's circuital law statement and mathematical form, comparison with Biot-Savart law, magnetic field due to cylindrical conductors (solid, thin hollow, thick hollow), infinite current sheet, solenoid (finite and infinite), and toroid.
3) Force on Moving Charges and Current-Carrying Conductors
Magnetic force on a moving charge (F = qvB sin theta), Lorentz force, trajectory of charged particles (straight line, circular, helical), cyclotron, velocity selector, Hall effect, force on a current-carrying conductor (F = BiL sin theta), force between parallel conductors, and standard equilibrium cases.
4) Torque on Current Loop and Moving Coil Galvanometer
Current loop as a magnetic dipole, magnetic moment M = NiA, torque on a loop in uniform field, work done in rotating a loop, potential energy of a magnetic dipole, moving coil galvanometer construction, deflection relation, current sensitivity, and voltage sensitivity.
Magnetic Effect of Current Download Notes & Weightage Plan
For each topic in the Magnetic Effect of Current chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Biot-Savart Law and Magnetic Field Configurations
Biot-Savart law, direction rules, field of straight wire, circular loop (centre and axis), arcs, Helmholtz coils, and concentric loops.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: B at centre of circular loop = mu_0 Ni / 2r; arc formula B = mu_0 i theta / 4pi r; infinite wire B = mu_0 i / 2pi r. These three cover most NEET questions.
- High-risk Area: Confusing the angle theta in Biot-Savart (angle between dl and r) with other angles in the problem geometry. Also forgetting the factor of N (number of turns) in the loop formula.
- Best Practice Style: Direct numerical application of standard results; finding resultant field at centre of concentric arcs.
Ampere's Circuital Law and Its Applications
Ampere's law statement, magnetic field in cylindrical conductors, solenoid, toroid, and infinite current sheet.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: B_solenoid = mu_0 n i (inside, uniform); B at end = half; toroid = mu_0 Ni / 2pi r; hollow cylinder B = 0 inside.
- High-risk Area: Assuming field inside a toroid is uniform like a solenoid. The toroid field varies as 1/r. Also forgetting that field outside an ideal solenoid is zero.
- Best Practice Style: Conceptual MCQs on B variation graphs; direct substitution numericals for solenoid and toroid.
Force on Moving Charges and Current-Carrying Conductors
Lorentz force, charged particle trajectories, cyclotron, velocity selector, force on wire, force between parallel conductors, Hall effect.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: r = mv/qB and T = 2pi m/qB (circular path); cyclotron frequency independent of speed; F/l = mu_0 i_1 i_2 / 2pi a and the definition of ampere.
- High-risk Area: Forgetting that magnetic force does zero work (kinetic energy unchanged). Confusing the four radius expressions. Applying wrong direction rule. Also: cyclotron cannot accelerate electrons (they require synchrotron due to relativistic mass increase).
- Best Practice Style: Numerical MCQs on radius and time period; conceptual MCQs on force direction; cyclotron energy calculation.
Torque on Current Loop and Moving Coil Galvanometer
Magnetic dipole moment, torque, work, potential energy, galvanometer principle, current sensitivity, voltage sensitivity.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: tau = NiAB sin theta; galvanometer S_i = NBA/C; the fact that radial field keeps theta = 90 in the galvanometer.
- High-risk Area: Confusing theta as the angle between the plane of the loop and B (it is the angle between the normal to the plane and B). Also: thinking voltage sensitivity always increases with current sensitivity (it does not, because resistance R also changes with N).
- Best Practice Style: Torque calculation MCQs; galvanometer sensitivity comparison when parameters change.
Magnetic Effect of Current Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Magnetic Effect of Current chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Swapping the angle definitions: In Biot-Savart, theta is the angle between the current element dl and the position vector r to the field point. In F = qvB sin theta, theta is between velocity v and magnetic field B. Students routinely substitute the wrong angle.
- Ignoring sin theta = 0 on the axis: On the line of a current-carrying wire, theta = 0 or 180 degrees, making dB = 0. Students forget this and attempt to compute a nonzero field on the wire axis.
A current element of length dl carrying current i has a field point P on its axis (theta = 0). Applying Biot-Savart: dB = (mu_0/4pi)(i dl sin 0 / r^2) = 0. Students who confuse this angle with some other geometry angle get a nonzero answer and pick the wrong option.
How NEET Frames The Trap
NEET distractors include computed values assuming theta = 90 degrees when the point is actually on the wire axis. Always verify: is the field point on the line of the current element?
Q. The magnetic field at a point on the axis of a current-carrying straight conductor is:
A. mu_0 i / 2pi r B. mu_0 i / 4pi r C. mu_0 i / 4pi r^2 D. Zero
Trick: On the axis of the wire, the angle between dl and r is 0 (or 180 degrees). Since sin 0 = 0, dB = 0 for every current element. The correct answer is D.
Mistake Snapshot (What Students Do Wrong)
- Using wrong radius expression for given data: Four forms exist: r = mv/qB = p/qB = sqrt(2mK)/qB = (1/B)sqrt(2mV/q). Students pick the momentum form when kinetic energy is given, or the velocity form when potential difference is given, leading to incorrect answers.
- Forgetting T is independent of v: Time period T = 2pi m / qB depends only on mass, charge, and field strength. Students incorrectly assume faster particles take less time per revolution.
A proton accelerated through 100 V enters a 0.1 T field perpendicular to its velocity. Using r = (1/B)sqrt(2mV/q): r = (1/0.1)sqrt(2 x 1.67 x 10^(minus 27) x 100 / 1.6 x 10^(minus 19)) = 0.0144 m. A student who uses r = mv/qB without first computing v from the potential difference will get stuck or make an error.
How NEET Frames The Trap
NEET gives the accelerating potential (not the velocity) and expects you to use r = (1/B)sqrt(2mV/q) directly. Distractors are computed using wrong substitutions.
Q. A proton (mass m, charge q) is accelerated through potential V and enters a magnetic field B perpendicular to its velocity. The radius of its circular path is:
A. mv / qB B. (1/B) sqrt(2mV/q) C. qBV / 2m D. sqrt(2qV) / mB
Trick: Kinetic energy = qV, so (1/2)mv^2 = qV gives v = sqrt(2qV/m). Substituting in r = mv/qB yields r = (m/qB)sqrt(2qV/m) = (1/B)sqrt(2mV/q). Answer is B.
Mistake Snapshot (What Students Do Wrong)
- Using angle of plane instead of normal: Torque tau = NiAB sin theta where theta is between the normal to the loop and B. If the plane makes angle alpha with B, then theta = 90 minus alpha. Students who directly use alpha get cos alpha instead of sin theta.
- Ignoring radial field in galvanometer: In a moving coil galvanometer the magnetic field is radial, so the plane of the coil is always parallel to B (theta = 90 degrees). Students sometimes substitute theta and get a sine factor when none exists.
A rectangular loop (N = 50, A = 0.04 m^2, i = 2 A) is placed in a 0.5 T field with the plane of the loop at 30 degrees to B. Theta (angle of normal with B) = 90 minus 30 = 60 degrees. Torque = 50 x 2 x 0.04 x 0.5 x sin 60 = 1.73 N m. Using sin 30 instead gives 1.0 N m, which is wrong.
How NEET Frames The Trap
NEET often states the angle of the plane (not the normal) with respect to B. The distractor is computed using sin of the given angle directly.
Q. A coil of 100 turns and area 0.01 m^2 carries 1 A in a 0.2 T field. The plane of the coil makes 60 degrees with B. The torque is:
A. 0.2 sin 60 = 0.173 N m B. 0.2 sin 30 = 0.1 N m C. 0.2 cos 60 = 0.1 N m D. 0.2 cos 30 = 0.173 N m
Trick: Plane at 60 degrees to B means the normal makes 90 minus 60 = 30 degrees with B. So tau = NiAB sin 30 = 100 x 1 x 0.01 x 0.2 x 0.5 = 0.1 N m. Answer is B.
Mistake Snapshot (What Students Do Wrong)
- Assuming S_V increases with N: Current sensitivity S_i = NBA/C increases with N. But voltage sensitivity S_V = NBA/RC = S_i/R. Since adding turns increases R proportionally, S_V may stay constant or even decrease. Students assume more turns always mean higher sensitivity for both.
- Confusing K with S_i: The galvanometer constant K = C/NBA is the inverse of current sensitivity. A smaller K means higher sensitivity. Students sometimes set K equal to S_i, inverting the relationship.
Doubling the number of turns N from 50 to 100: S_i doubles (from NBA/C to 2NBA/C). But the coil resistance R also roughly doubles. So S_V = S_i/R stays approximately unchanged. Students who pick 'S_V doubles' lose the mark.
How NEET Frames The Trap
NEET asks what happens to voltage sensitivity when N is doubled. The expected wrong answer is that it doubles (just like current sensitivity).
Q. If the number of turns in a moving coil galvanometer is doubled keeping all other parameters same, the voltage sensitivity:
A. Doubles B. Halves C. Remains same D. Becomes four times
Trick: S_V = NBA/RC. Doubling N doubles both the numerator (NBA) and the denominator (R, since resistance is proportional to N for the same wire). So S_V remains approximately unchanged. Answer is C.